Modern cable television systems (CATV) transmit television information from a headend downstream to the subscriber and receive data signals upstream from the subscriber to the headend. In a 70 channel system, the downstream signals typically occupy the band from 50-500 MHz. In addition, digital channels may be transmitted from 500-750 MHz. The upstream signals utilize the unoccupied band from approximately 5-50 MHz.
FIG. 1 illustrates a known cable distribution system 100. In the known system, the signal is transmitted from the headend 102 through a fiber optic cable 104 to a fiber node 106. At the fiber node the signal is converted to an electrical signal and distributed over coaxial cable 108 which contains a plurality of taps 110 connecting a plurality of residential units 114 to the distribution cable 108 via a drop cable 112. In older systems, the signal may be transmitted from the headend directly onto the coaxial cable 108. As used herein, the term cable distribution system refers to the distribution cable or fiber optic cable from the headend to the residential unit, sometimes known as the distribution loop plant. In each residential unit, there is a cable distribution system, referred to herein as the subscriber distribution system, which may include a portion installed by the cable company and a portion installed by the subscriber. The portion installed by the subscriber is typically installed by non-skilled persons and often utilizes cable and other components of inferior quality to that provided by the cable television company.
The channel capacity for the upstream signals is lower than can optimally be provided by the 45 MHz bandwidth due to interference signals on the lines. This results in approximately 30 percent reduction in the upstream channel capacity. It is known that these interference signals are off-the-air radio signals. Common radio interference sources are international shortwave stations and local amateur radio operators. International shortwave radio has frequency bands at 6 MHz, 7.5 MHz, 9.5 MHz and 12 MHz, for example. There are usually tens of radio stations active at each frequency band. Amateur radio has a group of similar frequency bands at 7 MHz, 10 MHz and 14 MHz, for example. Amateur radio signals may employ single sideband modulation that makes the transmit power non-stationary.
The field strength of international shortwave radio stations is typically 10-70 dB .mu.V/m. Short-wave radio interference could come from anywhere on the globe. An individual amateur radio transmitter could generate a field strength of 140 dB .mu.V/m at a distance of 10 meters. The field strength becomes weaker as the distance increases; being reduced to 100 dB .mu.V/m at a distance of 1000 meters.
It is believed that the interference on the upstream signal is caused by pickup from the cable distribution system, which is "funneled" into the system. The "funneling" is the result of the widespread cable lines that are all connected to the fiber node and then to the headend. The interference signals picked up by both the drop cables and distribution cables then add on the distribution cable. Thus, the interference at the fiber node will be greater than interference at any other portion in the coaxial cable distribution portion of the cable television system, and thus reduce the channel capacity of the upstream signals. It has also been shown that problems in the installation of the subscriber distribution system, caused both by use of inferior components and by the installation by non-skilled persons, results in this being a significant source of interference signals.
Cable television systems will become increasingly interactive. Assuming that the channel capacity is currently reduced by 30 per cent, a 10 dB reduction in noise should result in a 45 megabit per second (Mbps) increase in the channel capacity, thereby allowing significantly more data to flow in the upstream direction, thus permitting the interactive services provided by the cable system to be expanded.